In fabricating certain electronic components (e.g., semiconductor chips or wafers) it is conventional to bond one end of a gold or aluminum wire to a metallized contact pad on a semiconductor chip and the other end of the wire to a corresponding metallized portion on a supporting substrate. Such a technique is normally accomplished by thermocompression or ultrasonic bonding. Although wire bonding is an effective method it requires two separate bonds and has a relatively high labor cost.
To overcome such problems it is known to form "solder bumps" on the metallized contact pads of the semiconductor chips which then may be placed in contact with the corresponding metallized portion of the substrate. The solder bump is then exposed to an elevated temperature to reflow the solder to form the bond. Such a technique is disclosed in U.S. Pat. No. 4,273,859 which issued on June 16, 1981 and is hereby incorporated by reference herein.
Advantageously, the solder bump approach provides a high level of hermeticity. In conjunction with the use of a passivation layer, reflowed solder terminals seal the bonding pads and provide an effective hermetic seal for the device. Additionally, these solder joints have been found to have high mechanical strength and are also quite ductile and are capable of absorbing stresses induced by thermal cycling. Because of the high strength and integrity of the resulting solder interconnections, device reliability has been reported to be higher than for wire-bonded interconnections.
It is desirable to deposit as much solder as possible on each bonding pad to provide good mechanical strength for the solder joint. It is also important to have solder bumps with a maximum height in order to provide as much distance as possible between the semiconductor chip on the substrate to which it is bonded. This space precludes shorts between the device and substrate and permits encapsulant to flow therebetween.
Deposition of solder using evaporation and electroplating techniques have been used to deposit large amounts of solder depending upon the amount of material evaporated or electroplated. Also, it is known to immerse the device in molten solder to form the bump. However, with immersion, the amount of solder deposited and hence the solder bump height appears to be limited by the bonding pad area wherein the larger bonding pad areas give rise to solder bumps of greater height. While solder immersion has been found to yield less than desirable bump heights, it is the least expensive method of deposition.
Accordingly, there is a need for a method of increasing the height of solder bumps formed when using solder immersion techniques.